Wittig Reaction
The Wittig Reaction is a very important method in organic chemistry for creating alkenes. It was discovered by Georg Wittig in 1954. In this reaction, a special compound called a phosphorus ylide reacts with an aldehyde or ketone to form an alkene. This makes the reaction very useful in synthetic chemistry.
An ylide is a molecule that has two neighbouring atoms carrying opposite charges, but both have complete octets. In the Wittig reaction, the ylide used is usually triphenylphosphonium ylide. Ylides are also called Wittig reagents.
Key Points
- Used to convert aldehydes and ketones into alkenes.
- Forms the double bond exactly at the same carbon where the carbonyl group was present.
- Uses phosphorus ylides as the nucleophile.
Preparation of Ylide
The ylide is prepared in two steps:
Step 1: Formation of Phosphonium Salt
Triphenylphosphine reacts with an alkyl halide by nucleophilic attack. This gives an intermediate called a phosphonium salt.
Step 2: Deprotonation
The phosphonium salt is treated with a strong base (like an organolithium reagent), which removes a proton, forming the ylide.
Mechanism of the Wittig Reaction
Step 1: Formation of Oxaphosphetane
The carbanion part of the ylide attacks the carbonyl carbon of an aldehyde or ketone. This produces a four-membered ring intermediate called oxaphosphetane.
Step 2: Formation of Alkene
The intermediate breaks apart by breaking C–O and C–P bonds, leading to the formation of:
- An alkene (main product)
- Triphenylphosphine oxide (side product)
Applications of the Wittig Reaction
This reaction is widely used in the synthesis of natural products and complex organic molecules. Some important applications include:
1. Formation of Exocyclic Methylene Groups
Useful in preparing methylene steroids and other biologically active molecules.
2. Preparation of β,γ-Unsaturated Acids
The Wittig reaction allows selective formation without unwanted rearrangements.
3. Synthesis of Natural Products
- β-carotene (pro-vitamin A)
- Vitamin A
4. Formation of Large Ring Compounds
Rings containing 5 to 16 carbons can be prepared using this reaction.
5. Synthesis of Vinyl Halides
A special reagent, Ph₃P=CHCl, helps in preparing vinyl halides.
6. Synthesis of Ethers
Methoxy-methylene triphenylphosphorane reacts with carbonyl compounds to give vinyl methyl ethers. These can be converted to aldehydes by hydrolysis.
Detailed Notes:
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